The present invention relates generally to a system for evaluating spine stability using medical imaging machines capable of producing axial tomographic or “transverse slice” images of a patient, and more generally to a patient support and software for evaluating rotation of lumbar vertebrae using such machines.
The structure of the normal lumbar intervertebral disc opposes the rotation of adjacent vertebrae in the axial plane because of the strong, obliquely oriented fibers in the annulus fibrosis in the periphery of the disk. With rupture of these fibers, secondary to a radial tear of the annulus fibrosis, the resistance to rotation in the axial plane is diminished markedly and excessive rotation can occur when a mild rotatory torque is applied to the spine. To correct this problem of hypermobility, physicians may choose to fuse the damaged vertebrae together by means of a surgical procedure eliminating the rotatory mobility and adding stability.
No satisfactory means has yet been described to measure the severity of the hypermobility to determine whether it warrants this extensive surgery. Flexion-extension radiographs of the spine (i.e., with the patient bending forward) have been used to detect abnormal motion of the spine in patients with back pain and suspected degenerative spinal instability. However these radiographs have poor sensitivity for hypermobility and poor specificity.
In biomechanical studies, a radial tear of the annulus fibrosus reduces the stiffness of the disc to axial rotatory forces by a factor of two or more while it affects the stiffness to flexion or extension marginally. Therefore, in theory abnormal mobility and decreased stiffness of the lumbar spine is detected more effectively by studying the motions of the vertebra subjected to an axial rotatory torque than to a flexion or extension torque.
An invasive means to measure axial rotations of the lumbar vertebra has been described (stereophotogrammetry). In stereophotogrammetry, radio opaque markers placed on portions of the vertebrae, radiographs are obtained in two projections and then the patient is subjected to axial rotation and a second series of radiographs is obtained. Triangulation from two radiographs taken at different angles is then used to determine rotation of the vertebra. The method is too invasive and cumbersome for routine clinical use.
Another possibility has been tried recently. An open design MR magnet has been used to measure rotation of lumbar vertebra with a patient in a sitting or standing position, but without a reliable means of measurement. The type of imaging magnet which allows the patient to sit or stand is not generally available. A technique adapted to the generally available type of CT and MR scanners would be clinically more useful. The feasibility of detecting axial rotations of the lumbar spine in patients in a supine position has been documented. A study was done on supine patients in a CT machine using cushions to rotate the patient. Measurements appeared to be adequate for determination of stability, but the method was difficult to reproduce and uncomfortable for patients.